This invention relates in general to thick film electric circuit elements and to methods for fabricating electronic circuits that include thick film capacitors and/or thick film cross-overs.
There are various fabrication techniques known for making hybrid circuits including thick film capacitors. One such well-known technique is set forth in U.S. Pat. No. 3,669,733 - Allington, issued June 13, 1972. In the fabrication technique set forth in that patent, patterns of circuit interconnections and various passive circuit components such as resistors and capacitors are printed in layers or mounted on a ceramic substrate to form a hybrid thick film circuit.
Other U.S. Patents related to the fabrication of electronic circuits are listed below. This list is not intended to be exhaustive, but rather only representative.
U.S. Pat. No. 4,238,662 - Lao (12/9/80) PA1 U.S. Pat. No. 4,238,661 - Braunlich (11/9/80) PA1 U.S. Pat. No. 4,061,584 - Girard et al (12/6/77) PA1 U.S. Pat. No. 4,007,296 - Ansell et al (2/8/77) PA1 U.S. Pat. No. 3,882,059 - Elderbaum (5/6/75) PA1 U.S. Pat. No. 3,864,159 - Field et al (2/4/75) PA1 U.S. Pat. No. 3,688,361 - Bonini (9/5/72) PA1 U.S. Pat. No. 3,665,269 - Wright et al (5/23/72) PA1 U.S. Pat. No. 3,617,834 - Rayburn (11/2/71) PA1 U.S. Pat. No. 3,609,483 - Smyly (9/28/71) PA1 U.S. Pat. No. 3,604,082 - McBrayer et al (9/14/71) PA1 U.S. Pat. No. 3,591,905 - Elderbaum (7/13/71) PA1 U.S. Pat. No. 3,582,729 - Girard (6/1/71) PA1 U.S. Pat. No. 3,565,570 - Dien (1/23/71) PA1 U.S. Pat. No. 3,544,434 - Giller (12/1/70) PA1 U.S. Pat. No. 3,480,421 - Allen (11/25/69) PA1 U.S. Pat. No. 3,456,170 - Hatch (7/15/69) PA1 U.S. Pat. No. 3,453,781 - Greenman III (7/8/69) PA1 U.S. Pat. No. 3,394,386 - Weller et al (7/23/68) PA1 U.S. Pat. No. 3,179,545 - Bowers (4/20/65) PA1 U.S. Pat. No. 3,123,497 - Barnes et al (3/3/64) PA1 U.S. Pat. No. 3,002,137 - Kahn et al (9/26/61)
The subject matter of these patents as well as that of the Allington '733 patent are incorporated herein by reference.
Utilizing known fabrication techniques certain difficulties arise in the mass production of electronic circuits. Screen printed capacitors formed by the printing of a lower conductor electrode, then printing one or two layers of dielectric material and finally printing an upper electrode conductor are somewhat delicate and therefore are subject to damage. In the mass production of hybrid thick film circuits including one or more capacitors on a substrate, many mechanical manipulations and processing steps are required to fabricate the complete circuit. The fabrication may include one or more high temperature firings other than for just the formation of the capacitors. In the handling of the circuits during manufacture, screen printed capacitors are likely to be damaged. This is particularly true when hybrid circuit substrates must be stacked (usually 2-3 feet in automated carriers) for mechanical manipulation. Such stacking abraids the materials printed onto the substrate thereby damaging the components. Therefore mechanical protection of the circuit must be provided.
Another problem arises when thick film capacitors are to be laser-trimmed to a predetermined precise capacitance value. When a precisely valued capacitor is required, it is desirable to form a thick film capacitor on the substrate and then trim it after the entire electronic circuit is constructed and aged. As the circuit ages, capacitance values tend to change. If the capacitors are laser trimmed they may shift in capacitance value as they age after laser trimming. Trimming a capacitance value prematurely, i.e. before complete fabrication of the circuit will result in the capacitor changing value as a result of the additional processing steps required for completion of the circuit, i.e. additional heating and cooling steps where mechanical manipulation will change the capacitance value. In order to protect the capacitor, it is known to use a protective sealant composition such as desired in the Allington '733 patent. However, the use of such a protective composition does not completely solve all of the operational problems.
Protective sealant compositions effectively seal off or isolate the capacitor from its surrounding environment. During the aging process when the circuit is setting for a period of time before the capacitors are laser trimmed, there develops a humidity non-equilibrium. In other words, the environment of the capacitor inside the sealant material becomes different from the environment outside of the sealant material. After the sealant material is cut by the laser trimming process, the environments equalize and the value of capacitance over time may change as a result of the environmental change.